Colour displays typically work according to one of two general principles of operation. A first principle is the transmissive display principle in which a transmissive display screen (e.g. liquid crystal display) is back-lit by a white-light illumination source. Red, green and blue filter elements within the display screen selectively block or transmit light from the back light to produce a colour display. A second principle is that of colour sequential display whereby a display element is illuminated sequentially with red, green and blue light either from a colour wheel spinning in front of a white light source or three separate LEDs arranged to generate red, green and blue light respectively.
In the latter case, when driving LEDs the forward bias voltage of each LED is controlled to remain largely stable (varying a little) and brightness/luminous output of the LED is controlled by controlling the current through the LED. This is because, in having a diode voltage/current characteristic, current in a driven LED is approximately an exponential function of forward bias voltage according to the Shockley diode equation, so a small voltage change will result in a large corresponding current change. However, if the voltage is too high, the corresponding current may rise above the maximum rating for an LED and potentially damage it. Therefore, it is important that the power source connected to an LED provides the correct current. LEDs are typically connected to constant-current power sources as a result of this driven by a driver/control circuit to ensure that appropriate voltages/currents are applied to the LED. This means that in a colour display that employs sequentially-driven LEDs (e.g. red, green and blue), a colour-dedicated driver/control circuit is required for each LED colour since LEDs designed to produce red light require forward bias voltages (current) which differ from those required to drive an LED designed to produce blue or green light—the same being true as between blue and green LED driving requirements. Thus, a control circuit adapted to drive a red LED is unsuitable for driving a blue or green LED, and vice versa, and a control circuit adapted to drive a blue LED is unsuitable for driving a green LED, and vice versa. This unsuitability is also driven by the energy of photons generated by an LED, which is given by hν, where h is Planck's constant and ν is the frequency of the photon. Generated blue light typically has a frequency of about ν=2.17 TeraHertz, green has ν=1.9 Terahertz and red has ν=1.61 TeraHertz. Blue photons are more energetic than green which are more energetic than red. This leads to widely varying power requirements for each type of colour LED and therefore the lowest energy LED device (red) must be driven at a much higher power than the green or blue LEDs, but as the forward voltage for a red LED is typically much lower than the forward voltages for green and blue LEDs, the amount of current required in each channel varies by a large factor. This adds much cost to the production of drivers for colour displays employing different colour LEDs, and also significantly increases the size and weight of the display circuitry as a whole—which is particularly disadvantageous in helmet-mounted or head-mounted displays.
The invention aims to provide an improved display apparatus using LEDs.